This application relates primarily to banded, axial-flow fans that are used in combination with a housing. We use the term banded axial-flow fan to describe a fan having a hub that rotates on an axis and has blades extending in an outwardly direction (for example, generally radially) from the hub to a circular band that connects the blade tips. Blade rotation tends to force air in the general direction of the fan axis.
We use the term fan and housing combination to describe such axial flow fans in combination with a housing (e.g., a shroud or a duct) that guides airflow entering and/or leaving the fan. By shroud, we mean an airflow guide shaped and positioned to guide airflow moving between the fan and a heat exchanger. An example of a banded shrouded fan that moves air through a heat exchanger is disclosed in Gray U.S. Pat. No. 4,548,548, which is hereby incorporated by reference. By duct, we mean an elongated flow path such a those used in heat, ventilation, and air conditioning systems.
In order to compare the invention to the prior art, a figure from the '548 patent is reproduced here as FIG. 1B. See also, Longhouse, U.S. Pat. No. 4,685,513.
There have been efforts to reduce the effects of undesired airflow recirculation experienced with fan and housing combinations. Airflow recirculation can be explained as follows. The fan generates a pressure difference between the upstream side of the fan (the side from which air is drawn) and the downstream side of the fan (the side of the fan which exhausts the airflow)--i.e., pressure is higher on the downstream side than on the upstream side. Airflow recirculation is the undesired flow of air from the downstream to the upstream side of the fan, resulting from this pressure difference. For banded fans in a housing, the recirculating airflow passes through the running clearance between the housing and the band.
One way to reduce recirculation is to reduce the fan-to-housing clearance. Another way to reduce recirculation is to incorporate labyrinth seals between the fan and the housing. In some cases these methods may present difficulties in economic manufacture.
Hauser U.S. Pat. No. 4,357,914 discloses a cooling fan for use with an annular radiator having baffle rings to reduce recirculation.